1. Field of the Invention
The present invention relates to a thread machining control method for numerically controlled machine tools, and a thread machining apparatus.
2. Description of the Related Art
A related art thread machining control method is adapted to rotate a spindle in accordance with a spindle rotational number command S, determines a movement quantity of a feed axis by multiplying a rotational angle of the spindle by a screw pitch command P after the spindle reaches a predetermined position M (generally called a “marker position”), and thereby drives and controls the spindle. In short, the feed axis is controlled as a slave shaft with a speed of the spindle used as a master.
The related techniques will now be described with reference to a block diagram of FIG. 1.
A spindle 6 is rotated and controlled via a spindle speed controller 5 on the basis of a spindle rotational number command S given by a processing program. A spindle position detecting section 8 detects a spindle position APA-S by using a detection signal outputted from a spindle position detector (PG; pulse generator) 7. When a thread machining command C is analyzed by subjecting a processing program to an analysis in a program analyzing section 1, a thread machining command C including the screw pitch data P is sent out to a thread machining control section 2. After a predetermined rotational angle (marker position) is attained on the basis of the spindle position APA-S outputted from the spindle position detecting section 8, the thread machining control section 2 starts a thread machining process. The thread machining control section 2 then determines a movement quantity ΔZ of the feed axis (Z-axis) which is obtained by multiplying a variation quantity ΔAPA-S of the spindle position APA-S by the screw pitch data P, and sends out the movement quantity ΔZ to a Z-axis position controller 3. The Z-axis position controller 3 positions and controls the Z-axis 4 on the basis of the inputted feed axis movement quantity ΔZ.
According to the above-described related techniques, the feed axis is controlled as a slave shaft with the spindle speed used as a master. Therefore, when a thread machining operation is carried out with the spindle driven with a predetermined rotational number, the spindle and Z-axis are moved with the speed of the spindle and a relative speed of the Z-axis kept in a predetermined relation, so that a thread machining process of a high processing accuracy can be conducted. However, when a screw cutter of a formed shape, such as a thread machining tool (bit) is used, cutting chatter is liable to occur in general, so that a stable cutting process cannot be carried out.
The processing techniques which eliminate such cutting chatter, and which are capable of carrying out a stable thread machining process by conducting the process while varying a rotational number of the spindle, have heretofore been known (Japanese Patent Application Laid-open No.49-105277 A). However, when the rotational number of the spindle is varied in a thread machining process, an acceleration/deceleration error of a slave shaft varies, and a phase error occurs in the relation between the position of the spindle and that of the feed axis, so that a thread machining accuracy markedly decreases. Therefore, the alteration of the rotational number of the spindle could not be carried out in the thread machining process (Japanese Patent Application No.2000-126991 A).
These problems will be described complementarily with reference to the time charts shown in FIGS. 2A and 2B.
When the spindle is operated at a rotational speed Sv1 as shown in FIG. 2A with the spindle then reaching a predetermined position (marker position), the feed axis is moved in accordance with a speed command Zv1 which is obtained by multiplying a rotational movement quantity of the spindle by a predetermined screw pitch. When the spindle is then moved at a speed Sv2 by changing the speed of the spindle represented by a rotational number thereof with the spindle then reaching a predetermined position (marker position) in the same manner, the feed axis is necessarily moved in accordance with a speed command Zv2 obtained by multiplying a rotational movement quantity of the spindle by a predetermined screw pitch.
A rise of the speed of the feed axis (Z-axis) shows that the feed axis starts being moved at a time “a” at which the spindle reaches a predetermined position (marker position) as shown in FIG. 2B. The acceleration processing of the feed axis (Z-axis) is carried out by an acceleration/deceleration constant Tz thereof toward the speed command Zv1 or Zv2. When the feed axis is accelerated toward the speed command Zv1 in the above process, a triangle “abc” represents a following delay quantity of the feed axis with respect to the position of the spindle, and, when the feed axis is accelerated toward the speed command Zv2, a triangle “ab′c′” represents the same. The above following delay quantity causes a difference in relation between the position of the spindle and that of the feed axis to occur, and consequently, a phase difference between the positions of leads of a screw to occur during a thread machining process. In short, when the speed of the spindle is varied, the speed of the feed axis also varies, so that the following delay quantity of the feed axis varies. This causes the accuracy of the thread machining pitch to be deteriorated.